Metal sheets, such as tin-free steels (TFS) and aluminum sheets, that are used as materials for metal cans for use as food cans are coated in order to improve corrosion resistance, durability, and weather resistance. However, applying a coating has problems of complicated baking, a long treating time, and emission of large amounts of solvents.
In order to solve these problems, instead of coated steel sheets, film-laminated metal sheets, which are manufactured by laminating a thermoplastic resin film on a heated metal sheet, have been developed and are industrially used as materials for food cans.
In addition to baseline characteristics, such as processability and adhesiveness, materials for food cans, for example, two-piece cans require various capabilities, such as deep drawability, adhesiveness after processing and retort treatment, corrosion resistance, and ease of design.
The functionality of film-laminated metal sheets may be expanded by (1) a method of expanding the functionality of the film itself by adding a modifier having a desired function to the film or (2) a method of applying a modifier having a desired function or a resin containing the modifier to the film without modifying the film itself.
The method (1) of directly adding the modifier to the film has high production efficiency and profitability when the film having a certain function is manufactured in large quantities. However, since the shape and contents of food cans vary widely, and the function required for food cans varies with the type of food can, this method is not appropriate. This is because a change in function to be provided to a film requires washing of a resin extruder, a casting drum, and a chill roll and a shutdown of the production line for a long time, which significantly decreases production efficiency. On the other hand, with the method (2) of applying a resin containing a modifier to the film, it is easy to change the function to be provided to the film. Thus, the method (2) can satisfy various demands of food cans. This is because a tank of a coating liquid containing a modifier can be washed and changed in a short time.
A method of applying a resin containing a modifier to a film is described in Patent Literature 1, for example. In Patent Literature 1, a resin layer containing an epoxy resin as a main component and containing a melamine resin, a blocked isocyanate compound, and a colorant is formed between a metal sheet and a film.
Although epoxy resins are reactive and have good adhesion to metal sheets, epoxy resins have poor deep drawability. Thus, epoxy resins cannot be used to manufacture films that can be used as materials for two-piece cans. When a resin-coated metal sheet described in Patent Literature 1 is formed into a draw and redraw can (DRD can), the epoxy resin cannot follow the elongational deformation in the height direction and restricts the deformation of the material. As a result, the material breaks in the drawing process.
Patent Literatures 2 to 5 disclose a method of applying a resin to a film in order to improve adhesion. Patent Literatures 2 to 5 describe a composite system of a polyester resin and an epoxy resin or a structure mainly composed of an epoxy resin. Thus, as Patent Literature 1, Patent Literatures 2 to 5 cannot be applied to two-piece cans because of poor deep drawability. Furthermore, examples described in Patent Literatures 2 to 5 did not describe can processability or deep drawability. Thus, it is clear that Patent Literatures 2 to 5 do not consider two-piece cans requiring deep drawing.